Ok, a few things to update you on.
I borrowed my friends speedtouch 585 version 6 yesterday afternoon. I plugged it in at 2pm and sync'd at 4850 which would give me a great 4mb speed like it used to with the netgear but with disconnections once a day in the evening.
At 7.46pm, i disconnected, and resyncd at 4020. Same as what happened with the netgear.
This afternoon, i resyncd my router which is connected to my secondry socket upstairs as usual, and got 4210 which is fine as i wont disconnect at any sync thats enough to give me 3.5mb, its if it goes over this (around 4500) that causes a disconnect. I then decided I would sync in the test socket to see what sync i would get - I got 4250 - which is nothing compared to what I synced at in the phone socket itself.
So, the speedtouch, and a filtered faceplate wont help me.